Current analysis devices, as are routinely used in analytics, forensics, microbiology and clinical diagnostics, are able to carry out a multiplicity of detection reactions and analyses with a sample. In order to be able to carry out a multiplicity of examinations in an automated manner, various automatically operating apparatuses for the spatial transfer of measurement cells, reaction containers and reagent containers are required, such as, e.g., transfer arms with gripper functions, transport belts or rotatable transport wheels, as well as apparatuses for transferring liquids, such as, e.g., pipetting apparatuses. The devices comprise a central control unit which, by means of appropriate software, is able to largely autonomously plan and work through the work steps for the desired analyses.
Many of the analysis methods used in such autonomously operating analysis devices are based on optical methods. Determining clinically relevant parameters, such as, e.g., the concentration or activity of an analyte, is often carried out by virtue of part of a sample being mixed with one or more test reagents in a reaction vessel, which may also be the measurement cell, as a result of which a biochemical reaction or a specific binding reaction, e.g., an antigen/antibody binding reaction, are started, which brings about a measurable change in an optical, or other, physical property of the test set-up.
In addition to spectrophotometry and turbidimetry, nephelometry is a widely used analysis method. Corresponding analysis devices therefore have corresponding photometric measurement apparatuses.
A photometric measurement apparatus comprises at least one light source and at least one photodetector. Typically, the arrangement of light source and photodetector is selected in such a way that the light emitted by the light source passes through a measurement cell arranged at a recording location and the light detector measures the light which leaves the measurement cell again.
Analysis devices in which the photometric measurement apparatus is moveable relative to the measurement cells or in which the measurement cells are moveable relative to the photometric measurement apparatus are finding increasing use. This is advantageous in that a measurement apparatus is able, as it were, to examine a multiplicity of samples simultaneously, which significantly increases the sample throughput.
EP-A1-2309251 describes an apparatus for the photometric examination of samples, in which a multiplicity of stationary measurement locations are arranged on a circular trajectory at a circular apparatus for receiving reaction vessels, while the photometric measurement apparatus is moveable on a horizontal trajectory about the vertical axis of the apparatus for receiving reaction vessels. Naturally, it is alternatively also possible for the photometric measurement apparatus to have a stationary embodiment and for the apparatus for receiving reaction vessels to be rotated about the vertical axis thereof.
In such photometric systems, in which the photometric measurement apparatus is moved relative to the measurement cell (or vice versa), at least one measured value is detected per revolution for each one of the measurement locations. What must be ensured here for the correct measured value detection is that each measurement location is fixed during each revolution and retrieved by the photometric measurement apparatus. To this end, the system comprises a physical reference location as a reference point, which defines an initial position for the relative movement between measurement apparatus and measurement locations. Then, the individual measurement locations are determined in the case of a known, constant rotational speed by virtue of measuring time intervals relative to the physical reference location. The measured time intervals can then be associated with specific measurement locations. By way of example, a physical reference location can be formed by a fork light barrier, which is passed through once during each revolution.
However, the accuracy of the determination of the physical reference location is restricted in practice by various factors, such as, e.g., interference or noise in the photoelectric sensor signal or an insufficiently homogeneous movement of the measurement apparatus or of the measurement locations, as occurs often, in particular, in the case of an actuation by stepper motors. An inaccurate determination of the physical reference location leads to an inaccurate determination of the measurement locations in the subsequent revolution, which in turn, as a consequence, results in a reduced accuracy of the measured value detection. This, in turn, can lead to completely invalid faulty measurements, which reduces the throughput of the measurement system.